The invention relates to a transmission system between at least a transmitter and at least a receiver via a transmission medium, likely to cause a frequency offset and an attenuation, said emitter comprising emitting means for emitting a sinusoidal signal of known frequency, and said receiver comprising receiving means for receiving said sinusoidal signal and estimation means for estimating said frequency offset from said received sinusoidal signal. The invention likewise relates to a receiver intended to be used in such a system, more particularly, a modem.
The invention also relates to a method of estimating the frequency offset caused on a carrier by a transmission medium, comprising a transmission step of a sinusoidal signal of known frequency. The invention also relates to a computer program implementing said frequency offset estimating method. The invention finally relates to a method of testing a transmission medium likely to cause a frequency offset, said method comprising steps of transmitting known test signals, receiving said test signal, and analyzing received test signals to derive properties of said medium therefrom.
The invention is applied to all the transmission systems that utilize one or various carriers and enable to transmit a sinusoidal signal at least at a given instant. The invention more particularly applies to modems, notably in accordance with Recommendation V34 of the CCITT.
U.S. Pat. No. 5,206,886 describes a multicarrier transmission system that includes estimation means for estimating the frequency offset introduced by the transmission medium. The estimate obtained is used at the beginning of the data transmission phase to initialize the adaptive algorithms for recovering the carrier of the receiver. The proposed estimation method is a complex adaptive method based on the transmission of two sinusoidal signals having known frequencies f1 and f2. Upon reception, these signals are separated by filtering and the phases xcfx861(t) and xcfx862(t) are calculated as a function of time (this calculation causes a calculation of the arctangent to be performed for each signal and each considered instant). The functions are then analyzed to obtain said estimate.
The invention notably has for its object to propose another, simpler method of estimating the frequency offset caused by the transmission medium.
This object is achieved with the transmission system as defined in the opening paragraph and characterized in that said estimation means comprise sampling means for sampling said received sinusoidal signal, phase shifting means for producing Π/2+p.2Π phase shifted samples of said received sinusoidal signal, where p is an integer, first calculation means for calculating said attenuation from at least two of said phase shifted samples, and second calculation means for calculating said frequency offset from at least two of said phase shifted samples and from the calculated attenuation.
The proposed method is particularly simple and thus cost-effective in terms of processing power. In an embodiment which is particularly easy to implement, first calculation means comprise means for calculating a mean value of an addition of the square value of two Π/2 phase shifted samples.
When the receiver comprises a reception filter, second calculation means use two Π/2+p.2Π phase shifted samples where p is chosen so that said two samples are decorrelated. In an advantageous embodiment, the second calculation means comprise means for calculating a mean value of a product of said samples so as to provide a trigonometric function of said offset. When the received signal is not filtered, it is advantageous to chose p=0, because in this case, the invention implies only a single calculation of a trigonometric function.
The invention likewise proposes another use of an estimate of said frequency offset. Indeed, according to the invention, a system as described in the opening paragraph, which comprises test means for testing said transmission medium notably by transmitting, receiving and then analyzing known test signals so as to derive therefrom properties of said medium, also includes correction means for correcting said offset during the reception of at least certain of said test signals.
It is thus possible to reduce the effect of the frequency offset on the modeling of the transmission medium, more particularly on the evaluation of the signal-to-noise ratio. This evaluation of the signal-to-noise ratio is notably used for choosing at the transmitter end a transmission rate on the transmission medium. The invention particularly enables to avoid that the frequency offset introduced by the medium is interpreted as noise, that is, that the signal-to-noise ratio is overrated and thus that the transmission rate is underrated.